1. Field of the Invention
The present invention generally relates to apparatus and methods for assembling pipes, and more particularly to methods and systems for joining tubes for solar receivers.
2. Description of Related Art
Solar thermal power plants may be used to obtain electric power from the sun. In such plants, the solar flux impinges on tubes through which a heat exchange medium flows. In some solar thermal power plants, tubes are situated in a solar collector, such as along the axis of a parabolic trough. The heated heat exchange medium from the tubes may be used in a thermodynamic cycle to generate electric power.
FIGS. 1-3 show a typical prior art tube 100 for use in solar thermal plants. Tubes 100 are sometimes referred to as “solar receiver tubes” or “heat collection elements (HCE).” Tube 100 may be, for example and without limitation, a SCHOTT solar receiver tube model PTR 70 (SCHOTT Solar, Inc., Albuquerque N. Mex.). Tube 100 having a length L includes an outer tube 110 having a diameter D capped at each end by a flange 115, an inner tube 111 coaxial with outer tube 110 and having a diameter d, and a bellows 113 that connects the flange and inner tube. Tubes 110 and 111, bellows 113 and flange 115 are sealed to form a volume 112, which is evacuated to provide a high thermal insulation between tubes 110 and 111.
In general, the length L is from 5 feet (1.5 m) to 20 feet (6 m), the diameter D is from 2 inches (50 mm) to 7 inches (0.18 m), and the diameter d is from 1 inch (25 mm) to 4 inches (0.1 m).
Typically, tube 110 is a glass tube, and tube 111, flange 115, and bellows 113 are metal. Tube 110 is generally transparent to sunlight to facilitate the solar heating of a heat exchange medium that may flow through tube 111, as indicated by arrows in FIG. 1.
In certain embodiments, tube 111 protrudes longitudinally beyond the end of each flange 115 by a distance S, which it typically from 0.375 inches (10 mm) to 4 inches (0.1 m). The portion of tube 111 that so protrudes is referred to as a collar 114. In certain other embodiments, solar energy systems are formed from multiple solar receiver tubes 100 by joining collars 114 of adjacent tubes. Collar 114 may includes an index, which may be the center line of the tube weld joint, which may be used to rotationally align adjacent tubes for welding.
Due their length, L, and glass components, solar receiver tubes tend to be fragile, and difficult to join, typically by welding, since the collars 114 protrude beyond the ends of the glass outer tube 110 by a relatively small distance from each end. Further, collars 114 are adjacent to bellows 113, on whose integrity the vacuum of volume 112 depends. Solar receiver tube are thus difficult to join without damaging the more fragile glass outer tube 110 or the bellows 113 joining tubes 110 and 111. There is a need in the art for methods and apparatus that permit the easy and rapid joining of such tubes to facilitate more efficient assembly of solar energy systems.